(Image credit: X-ray: NASA/CXC/Swinburne Univ. of Technology/A. Graham et al.; Optical: NASA/ESA/STScI)

Data from NASA’s Chandra X-ray Observatory and the Hubble Space Telescope describe how a black hole could have been delivered to spiral galaxy NGC 4424 by a smaller one.

NGC 4424 is located about 54 million light-years from Earth in the Virgo cluster of galaxies. The main panel of this previously published image shows a wide-field view of this galaxy in Hubble optical light. The image is about 45,000 light-years across. The center of this galaxy is expected to host a large black hole estimated to contain a mass of between 60,000 and 100,000 Suns. There are also likely to be millions of stellar-mass black holes, containing between 5 and 30 solar masses, scattered throughout the galaxy.

Inset features a close-up view of NGC 4424 showing X-ray data from Chandra (blue), plus infrared data from Hubble (red) with infrared light subtracted from a model of NGC 4424 in the image to show other faint features. This inset image is about 1,160 light-years across.

The elongated red object is a cluster of stars that the authors of the new study have dubbed “Nikhuli,” a name associated with the Tulini festive period of celebration and wish for a rich harvest. This name is taken from the Sumi language of the Indian state of Nagaland. The Chandra data show a point source of X-rays, the observatory reports in a statement.

The researchers determined that Nikhuli is likely the center of a small galaxy that has had most of its stars stripped away by colliding with the larger galaxy NGC 4424. Nihuli has also been stretched out by gravitational forces as it falls toward the center of NGC 4424, giving it an elongated shape. Currently, Nikhuli is about 1,300 light-years from the center of NGC 4424, or about 20 times closer than Earth to the Milky Way’s giant black hole.

One possible explanation for the inset Chandra X-ray source is that Nikhuli matter is rapidly falling into a stellar-mass black hole. However, because such smaller black holes are expected to be rare in a Nikhuli-sized cluster, the authors argue that material is more likely to slowly fall onto a more massive black hole weighing between 40,000 and 150,000 Suns. This is similar to the expected size of the black hole at the center of NGC 4424.

These results imply that Nikhuli is likely acting as a delivery system for NGC 4424’s supply of black holes, in this case bringing a massive one. If the center of NGC 4424 contains a massive black hole, the Nikhuli massive black hole should end up orbiting it. The distance separating the pair should shrink until gravitational waves are produced and the two massive black holes merge with each other.

An article describing these results appeared in the December 2021 issue of The Astrophysical Journal.